Near-Infrared Hubble Space Telescope Polarimetry of a Complete Sample of Narrow-Line Radio Galaxies
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MNRAS 444, 466–475 (2014) doi:10.1093/mnras/stu1390 Near-infrared Hubble Space Telescope polarimetry of a complete sample of narrow-line radio galaxies E. A. Ram´ırez,1‹ C. N. Tadhunter,2 D. Axon,3,4† D. Batcheldor,5 C. Packham,6 E. Lopez-Rodriguez,6 W. Sparks 7 and S. Young8 1Universidade de Sao˜ Paulo, IAG, Rua do Matao˜ 1226, Cidade Universitaria,´ Sao˜ Paulo 05508-900, Brazil 2Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK 3Physics Department, Rochester Institute of Technology, Rochester, NY 14623, USA 4School of Mathematical and Physical Sciences, University of Sussex, Brighton BN1 9QH, UK 5Department of Physics and Space Sciences, Florida Institute of Technology, FL 32901, USA 6Department of Physics & Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA 7Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD21218, USA 8Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB, UK Accepted 2014 July 7. Received 2014 June 30; in original form 2014 April 10 ABSTRACT We present an analysis of 2.05 µm Hubble Space Telescope polarimetric data for a sample of 13 nearby Fanaroff–Riley type II (FRII) 3CR radio sources (0.03 <z<0.11) that are classified as narrow-line radio galaxies (NLRG) at optical wavelengths. We find that the compact cores of the NLRG in our sample are intrinsically highly polarized in the near-infrared (near-IR) (6 <P2.05 µm < 60 per cent), with the electric vector (E-vector) perpendicular to the radio axis in 54 per cent of the sources. The levels of extinction required to produce near-IR polarization by the dichroic extinction mechanism are consistent with the measured values recently reported in Ram´ırez et al., provided that this mechanism has its maximum efficiency. This consistency suggests that the nuclear polarization could be due to dichroic extinction. In this case, toroidal magnetic fields that are highly coherent would be required in the circumnuclear tori to align the elongated dust grains responsible for the dichroic extinction. However, it is not entirely possible to rule out other polarization mechanisms (e.g. scattering, synchrotron emission) with our observations at only one near-IR wavelength. Therefore, further polarimetry observations at mid-IR and radio wavelengths will be required to test whether all the near-IR polarization is due to dichroic extinction. Key words: techniques: polarimetric – galaxies: active – infrared: galaxies. These detections are attributed to light from the broad-line region 1 INTRODUCTION scattered by material out of the plane of the torus. The orientation-based unified schemes for active galactic nuclei Optical polarimetric imaging of Fanaroff and Riley type II (FRII; (AGN; Antonucci 1993; Urry & Padovani 1995) propose that the Fanaroff & Riley 1974) PRG also shows a centrosymmetric pattern quasar continuum light in type-2 radio galaxies is hidden from our of polarization vectors contained within a bi-cone structure in seven direct view by an obscuring torus at optical wavelengths. out of eight NLRG in the Cohen et al. (1999) sample, including In the case of powerful radio galaxies (PRG), polarimetry obser- Cygnus A (see also Tadhunter, Scarrott & Rolph 1990;Ogleetal. vations provide strong evidence to support the unified schemes. For 1997). This, together with the detection of broad polarized Hα lines, instance, optical spectropolarimetric observations of narrow-line reveals the presence of broad-cone reflection nebulae illuminated radio galaxies (NLRG) show broad Hα lines detected in a signifi- by the AGN. cant number of objects (Antonucci 1984; Goodrich & Cohen 1992; The direct detection of the unresolved AGN in NLRG is diffi- Tran, Cohen & Goodrich 1995;Ogleetal.1997;Cohenetal.1999). cult at optical wavelengths, given the high extinction and the ev- ident presence of dust (de Koff et al. 1996; Chiaberge, Capetti & Celotti 1999, 2000, 2002). However, Hubble Space Telescope (HST) E-mail: [email protected] observations of Cygnus A show a clear progression from a dou- † Deceased. ble V-shape structure detected at optical and shorter near-infrared C 2014 The Authors Downloaded from https://academic.oup.com/mnras/article-abstract/444/1/466/1010766/Near-infrared-Hubble-Space-Telescope-polarimetryPublished by Oxford University Press on behalf of the Royal Astronomical Society by University of Sheffield user on 17 October 2017 NIR HST polarimetry of FRII NLRG 467 (near-IR) wavelengths, to a compact nuclear source at longer near- (Cygnus A) extracted from the Mikulski Archive for Space Tele- IR wavelengths (Djorgovski et al. 1991; Tadhunter et al. 1999). scopes. They are included because these are the only other 3C PRG More recently, we detected a point-like nucleus at 2.05 µmin at similar redshifts observed with HST/Near Infrared Camera and 80 per cent of a nearby sample of NLRG (Ram´ırez et al. 2014), sug- Multi-Object Spectrometer (NICMOS) at 2.05 µm in a similar way gesting that the central engine is shining directly through a dusty to our complete sample. These three additional objects, along with torus in these sources, providing evidence for the unified schemes. the 10 objects in the main sample, comprise an ‘extended sample’ It is not only infrared (IR) imaging and spectroscopic observa- of 13 3CR radio galaxies, all located in the Northern hemisphere. tions that are useful in the study of the central obscuring region in Dedicated HST polarimetric observations were obtained with PRG: the analysis of near-IR polarimetric data also has the potential the Near Infrared Camera and Multi-Object Spectrometer 2 (NIC- to provide important complementary information on the geometry MOS2), through the three long polarimeter (POL-L) filters, centred and intrinsic properties of the central regions. Studies at the K-band at 2.05 µm. The principal axes of the three polarizers are offset of the nuclear polarization of Centaurus A and Cygnus A show high by ∼60◦ from each other (Viana et al. 2009). NICMOS2 has a core polarizations of 9 ± 1 and 28 ± 1 per cent, respectively, with field of view of 19.2 × 19.2 arcsec. The observations of the com- the electric vector (E-vector) position angle (PA) aligned perpen- plete sample were made during Cycle 13, between 2005 April and dicular to the radio jet (Bailey et al. 1986; Tadhunter et al. 2000). 2006 June (GO 10410, principal investigator, PI: Tadhunter). The A similar configuration is found in 3C 223.1 and 3C 433 at 2.2 observations were executed in multiple accumulate mode.2 and 2.05 µm, respectively: polarization E-vectors perpendicular to The observations for GO 10410 were taken with the standard the radio jet axis, in addition to dust lanes orientated perpendicu- X-STRIP-DITHER-CHOP pattern (Viana et al. 2009). The spacing of the lar to the radio jets (Antonucci & Barvainis 1990;Ram´ırez et al. chop in the Y direction was 31.5 to 35.5 arcsec, and the dither spacing 2009). This is consistent with a dichroic1 or scattering origin for the in the X direction was 0.9 arcsec. A total on-galaxy integration time polarization, since such mechanisms can produce polarization PAs of 1024 s (4 dithers of 256 s each: ∼0.5 orbits in total) for each POL- that are perpendicular to the radio jet orientation. In contrast, the L filters was achieved, with the exception of the relatively bright polarization PAs for any synchrotron emission from the inner jets source 3C 433, for which shorter exposure times and more dithers tend to be parallel to the radio jet orientation (e.g. Lister 2001). were used in order to avoid saturation (4 dithers of 16 s each, and 12 Our detailed study of 3C 433 (Ram´ırez et al. 2009) suggested that dithers of 64 s each, for each POL-L filters; more details in Ram´ırez the near-IR polarization of the core (8.6 ± 1.0 per cent) is produced et al. 2009). Similar off-galaxy background exposures were used to by highly efficient dichroic extinction (AV ≈ 15 mag). Moreover, the facilitate accurate background subtraction without compromising polarized core in Cygnus A (28 ± 1 per cent; Tadhunter et al. 2000), the signal-to-noise ratio in the nuclear regions of the galaxies. could also be explained in terms of in a similar fashion. In addition For 3C 293 and 3C 305 we used archive data from programme to the detection of highly polarized compact core sources, an in- GO 7853 (PI: Jackson). These data were taken on 1998 August teresting extended polarization pattern has been detected along one 19, using NICMOS2 with the POL-L filters at 2.05 µm, and were edge of the bi-cone detected at 2.0 µm in Cygnus A (Tadhunter et al. made by executing a NIC-SPIRAL-DITH pattern with 2 points in steps 2000), providing potentially valuable information on the anisotropic of 3.5 arcsec (Viana et al. 2009), achieving an on-galaxy exposure illumination by the AGN and the properties of the obscuring torus time of 2176 s per POL-L filter. structure. The detected polarization pattern in Cygnus A (Tadhunter For 3C 405, the observations were taken from programme GO et al. 2000) has been explained in terms of the illumination of 7258 (PI: Tadhunter; see details in Tadhunter et al. 1999, 2000). the extended structures by anisotropic near-IR radiation emitted by These observations were executed on 1998 December 16 in the same the outer parts of a warped accretion disc (Sanders et al. 1989; fashion as the GO 10410 programme: using coordinated parallel Tadhunter et al. 2000). The detection of similar patterns in other exposures observations as part of a chop process.